1. Field of the Invention
The invention relates to a method and apparatus for measuring a cross sectional area and other dimensional properties of elongated conducting material and more particularly to a radio-frequency magnetic measuring method and apparatus for determing the area and weight per unit length of elongated material. The method and apparatus is useful in the measurement of solid and hollow conducting bar type products such as steel reinforcing bars as well as solid and hollow shaped bars of steel, aluminum, brass, copper and other conductors as they are manufactured.
2. Prior Art
In the field of nondestructive testing by magnetic means, it is known to use a primary solenoid for inducing a magnetic field in solid and hollow type materials and then measuring the extent and properties of such induction by a secondary coil enclosing or located near the primary material. Such measurements rely on eddy current effects within the bar type material which set up a secondary alternating current magnetic field in the secondary coil. The phenomenon is explained in the book, "Nondestructive Testing Handbook" by R. C. McMaster, Ronald Press (1959), Section 36, incorporated by reference herein. Patents which show prior art methods and apparatus for magnetic testing of bar type materials include U.S. Pat. Nos. 2,097,947; 2,555,853; 2,649,573; 3,142,796; and 3,560,845. The methods and apparatus described in the aforementioned book and patents relate to systems wherein the magnetic field penetrates the bar type material to generate eddy currents, whose influence is measured by external measuring apparatus.
In such systems the magnetic field strength as well as variabilities in the permeability and conductivity of the material may severely influence the measurement being made. In prior art systems, eddy currents which are set up generate magnetic fields which are out of phase with the main signal impressed on the primary solenoid. Such out of phase signals may be employed to detect defects such as cracks and occlusions in the material, as well as variations in permeability and conductivity.
It is an object of the present invention to provide a method and apparatus for detecting dimensional properties of elongated electrically conducting materials by means of a magnetic nondestructive testing apparatus in which a magnetic field indicative of the dimensional properties of the elongated material is in phase with the magnetic field inducing it, thereby eliminating reliance on eddy current effects.